Ni-base alloy for turbine rotor of steam turbine and turbine rotor of steam turbine

ABSTRACT

An Ni-base alloy for a turbine rotor of a steam turbine contains in percent by weight C: 0.05 to 0.15, Cr: 22 to 28, Co: 10 to 22, Mo: 8 to 12, Al: 0.8 to less than 1.5, Ti: 0.1 to 0.6, B: 0.001 to 0.006, Re: 0.1 to 2.5, and the balance of Ni and unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2008-092782 filed on Mar. 31,2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a material configuring a turbine rotorof a steam turbine into which high-temperature steam flows as a workingfluid, and more particularly to an Ni-base alloy for a turbine rotor ofa steam turbine excelling in high-temperature strength and the like, anda turbine rotor of a steam turbine made of the Ni-base alloy.

2. Description of the Related Art

For a thermal power plant including a steam turbine, a technology forsuppression of the emission of carbon dioxide is being watched withinterest in view of the global environmental protection, and needs forhighly efficient power generation are increasing.

To increase the power generation efficiency of the steam turbine, it iseffective to raise the turbine steam temperature to a high level, andthe recent thermal power plant having the steam turbine has its steamtemperature raised to 600° C. or higher. There is a tendency that thesteam temperature will be increased to 650° C., and further to 700° C.in future.

A turbine rotor, in which moving blades rotated by high-temperaturesteam are implanted, has a high temperature by circulation ofhigh-temperature steam and generates a high stress by rotating.Therefore, the turbine rotor is required to withstand a high temperatureand a high stress, and a material configuring the turbine rotor isdemanded to have excellent strength, ductility and toughness in a rangeof room temperature to a high temperature.

Particularly, if the steam temperature exceeds 700° C., a conventionaliron-based material is poor in high-temperature strength, so that theapplication of the Ni-base alloy is considered in for example JP-A7-150277(KOKAI).

The Ni-base alloy has been applied extensively as a material mainly forjet engines and gas turbines because it is excellent in high-temperaturestrength and corrosion resistance. As its typical examples, Inconel 617alloy (manufactured by Special Metals Corporation) and Inconel 706 alloy(manufactured by Special Metals Corporation) have been used.

As a mechanism to enhance the high-temperature strength of the Ni-basealloy, Al and Ti are added to secure the high-temperature strength byprecipitating a precipitated phase called as a gamma prime phase(Ni₃(Al, Ti)) or a gamma double prime phase, or both of them within themother phase material of the Ni-base alloy. There is for example Inconel706 alloy which secures high-temperature strength by precipitating boththe gamma prime phase and the gamma double prime phase.

Meanwhile, the high-temperature strength of Inconel 617 alloy is securedby reinforcing (solid-solution strengthening) the mother phase of Nigroup by adding Co and Mo. For example, JP-A 2002-88455(KOKAI) and JP-A2001-247942(KOKAI) disclose a Ni-base alloy having high-temperaturestrength characteristic improved by adjusting added element componentsbased on the components of Inconel alloy. The Ni-base alloy of JP-A2002-88455(KOKAI) is provided with improved sulfidation corrosion at ahigh temperature. JP-A 2001-247942(KOKAI) describes a rotor shaft usinga Ni-base alloy which suppresses a fragile intermetallic compound formedwhen used for a long time.

Since the above-described conventional Ni-base alloys are poor inproductivity, they were used for relatively small high-temperature partsand the like only.

Therefore, in a case where the conventional Ni-base alloy is applied to,for example, jet engine or gas turbine members, portions where theNi-base alloy is used are limited to small blades having a length ofless than 1 m, a disk material having a gross weight of less than 1 tonor the like.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides an Ni-base alloy for aturbine rotor of a steam turbine that workability such as forgeabilityis excellent, and a large-size forged product turbine rotor can beproduced, and a turbine rotor of a steam turbine.

According to an aspect of the invention, there is provided an Ni-basealloy for a turbine rotor of a steam turbine, which contains in percentby weight C: 0.05 to 0.15, Cr: 22 to 28, Co: 10 to 22, Mo: 8 to 12, Al:0.8 to less than 1.5, Ti: 0.1 to 0.6, B: 0.001 to 0.006, Re: 0.1 to 2.5,and the balance of Ni and unavoidable impurities.

According to an aspect of the invention, there is also provided aturbine rotor which is disposed through a steam turbine into whichhigh-temperature steam is introduced, wherein at least a predeterminedportion is formed of the Ni-base alloy for the turbine rotor of a steamturbine described above.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described below.

An Ni-base alloy for a turbine rotor of a steam turbine in an embodimentaccording to the present invention is composed of the compositionalcomponent ranges shown below. In the following description, percentagesindicating the compositional components are by weight unless otherwiseindicated.

(Ml) Ni-base alloy which contains C: 0.05% to 0.15%, Cr: 22% to 28%, Co:10% to 22%, Mo: 8% to 12%, Al: 0.8% to less than 1.5%, Ti: 0.1% to 0.6%,B: 0.001% to 0.006%, Re: 0.1% to 2.5%, and the balance of Ni andunavoidable impurities.

In the unavoidable impurities in the Ni-base alloy of the above (Ml), itis preferably suppressed that at least Si is 1% or less, and Mn is 1% orless.

The Ni-base alloy having the compositional component ranges describedabove is suitable as a material configuring a turbine rotor of a steamturbine which has a temperature in a range of 680 to 750° C. during itsoperation. All portions of the turbine rotor of the steam turbine may bemade of the Ni-base alloy, and some portions, which have a particularlyhigh temperature, of the turbine rotor of the steam turbine may be madeof this Ni-base alloy. As some portions of the turbine rotor of thesteam turbine which have a high temperature, there are specifically allregions of a high-pressure steam turbine section, or regions rangingfrom the high-pressure steam turbine section to some portions of anintermediate-pressure steam turbine section.

The Ni-base alloys of the compositional component ranges described abovecan improve workability such as forgeability. In other words, theNi-base alloy is used to configure the turbine rotor of the steamturbine, so that the workability such as forgeability of the turbinerotor can be improved, and the turbine rotor having high reliability canbe produced without generating a crack or the like when manufacturing.

The reasons of limiting the individual compositional component ranges ofthe Ni-base alloy according to the present invention described abovewill be described below.

(1) C (Carbon)

C is useful as a component element of M₂₃C₆ type carbide which is astrengthening phase, and particularly, the creep strength of the alloyis maintained by precipitating the M₂₃C₆ type carbide during theoperation of the steam turbine in a high-temperature environment of 650°C. or higher. And, it also has an effect of securing the fluidity of amolten metal at the time of casting. If the C content is less than0.05%, a sufficient precipitation amount of carbide cannot be secured,so that mechanical strength is degraded, and the fluidity of the moltenmetal at the time of casting lowers considerably. Meanwhile, if the Ccontent exceeds 0.15%, the tendency of segregation of componentsincreases at the time of producing a large ingot, the generation of M₆Ctype carbide which is an embrittlement phase is promoted, and mechanicalstrength is improved, but forgeability is degraded. Therefore, the Ccontent is determined to be 0.05% to 0.15%.

(2) Cr (Chromium)

Cr is an indispensable element to improve oxidation resistance,corrosion resistance and mechanical strength of the Ni-base alloy.Besides, it is indispensable as a component element of the M₂₃C₆ typecarbide, and particularly, the creep strength of the alloy is maintainedby precipitating the M₂₃C₆ type carbide during the operation of thesteam turbine in a high-temperature environment of 650° C. or higher.And, Cr improves the oxidation resistance in a high-temperature steamenvironment. If the Cr content is less than 22%, the oxidationresistance decreases. Meanwhile, if the Cr content exceeds 28%,precipitation of the M₂₃C₆ type carbide is accelerated considerably,resulting in increasing the tendency of coarsening. Therefore, the Crcontent is determined to be 22% to 28%.

(3) Co (Cobalt)

In the Ni-base alloy, Co improves the mechanical strength of a motherphase by forming a solid solution in the mother phase. But, if the Cocontent exceeds 22%, an intermetallic compound phase which degrades themechanical strength is generated, and forgeability is degraded.Meanwhile, if the Co content is less than 10%, workability is degraded,and the mechanical strength is lowered. Therefore, the Co content isdetermined to be 10% to 22%.

(4) Mo (Molybdenum)

Mo provides an effect of forming a solid solution into an Ni motherphase to enhance the mechanical strength of the mother phase, and itspartial substitution in M₂₃C₆ type carbide enhances the stability of thecarbide. If the Mo content is less than 8%, the above effect is notexerted, and if the Mo content exceeds 12%, a tendency of segregation ofcomponents increases when a large ingot is produced, and the generationof M₆C type carbide which is an embrittlement phase is accelerated.Therefore, the Mo content is determined to be 8% to 12%.

Mo is common with the above-described Co on a point that they have aneffect to improve the mechanical strength of the mother phase. And, toexhibit effectively the common characteristic and their othercharacteristics, when the Mo content is for example 8 to less than 10%,it is desirable that the Co content is larger than 15% and not largerthan 22%. When the Mo content is for example 10 to 12%, it is desirablethat the Co content is 10% to 15%.

(5) Al (Aluminum)

Al generates a γ′ phase (gamma prime phase: Ni₃Al) with Ni and improvesthe mechanical strength of the Ni-base alloy based on the precipitation.If the Al content is less than 0.8%, the mechanical strength is notimproved in comparison with a conventional steel, and if the Al contentis 1.5% or more, the mechanical strength is improved, but forgeabilityis degraded. Therefore, the Al content is determined to be 0.8% to lessthan 1.5%.

(6) Ti (Titanium)

Similar to Al, Ti generates a γ′ phase (gamma prime phase: Ni₃Ti) withNi and improves the mechanical strength of the Ni-base alloy. If the Ticontent is less than 0.1%, the above effect is not exerted, and if theTi content exceeds 0.6%, hot workability is degraded, and notchsensitivity becomes high. Therefore, the Ti content is determined to be0.1% to 0.6%.

(7) B (Boron)

B segregates in the grain boundary to affect the high-temperaturecharacteristics. And, B has an effect to improve the mechanical strengthof an Ni mother phase by precipitating in the mother phase. If the Bcontent is less than 0.001%, the effect to improve the mechanicalstrength of the mother phase is not exerted, and if the B contentexceeds 0.006%, there is a possibility that the grain boundary isembrittled. Therefore, the B content is determined to be 0.001% to0.006%.

(8) Re (Rhenium)

Re has an effect to improve the mechanical strength of an Ni motherphase by forming a solid solution in the mother phase. If the Re contentis less than 0.1%, an effect to improve the mechanical strength of themother phase is not exerted, and if the Re content exceeds 2.5%, afragile phase is formed. Therefore, the Re content is determined to be0.1% to 2.5%.

Similar to the Re, Co and Mo have an effect to improve the mechanicalstrength of the Ni mother phase by forming a solid solution in themother phase. But, when the content is same, the Re is most excellent inimprovement of the mechanical strength and can improve the mechanicalstrength without largely varying the chemical component composition of abase metal.

(9) Si (Silicon), Mn (Manganese), Cu (Copper), Fe (Iron) and S (Sulfur)

Si, Mn, Cu, Fe and S are classified to unavoidable impurities in theNi-base alloy according to the present invention. The residual contentsof the unavoidable impurities are desired to be decreased toward 0% asmuch as possible. It is desirable that at least Si and Mn in theunavoidable impurities are suppressed to 1% or below.

Si is added to the ordinary steel to supplement the corrosionresistance. But, since the Ni-base alloy has a large Cr content tosecure sufficient corrosion resistance, a residual content of Si in theNi-base alloy according to the present invention is determined to be 1%or less, and it is desired that the residual content is reduced to 0% asmuch as possible.

In the ordinary steel, Mn prevents brittleness, which results from S(sulfur), in a form of MnS. But, since the S content in the Ni-basealloy is very small, it is not necessary to add Mn. Therefore, theresidual content of Mn in the Ni-base alloy according to the presentinvention is determined to be 1% or below, and it is desired that theresidual content is reduced to 0% as much as possible.

The above-described Ni-base alloy according to the present invention isproduced by melting the compositional components configuring the Ni-basealloy by a vacuum induction melting furnace, subjecting the obtainedingot to a soaking treatment, forging it, and conducting a solutiontreatment.

It is preferable that the soaking treatment is maintained at atemperature range of 1050 to 1250° C. for 5 to 72 hours, and thesolution treatment is maintained at a temperature range of 1100 to 1200°C. for 4 to 5 hours. Here, the solution treatment temperature isdetermined to form a homogeneous solid solution of the γ′ phaseprecipitates, and if the temperature is lower than 1100° C., a solidsolution is not formed adequately. If the temperature exceeds 1200° C.,crystal grains are coarsened and the strength is degraded. And, forgingis performed at a temperature range of 950 to 1150° C.

In a case where the above-described Ni-base alloy according to thepresent invention is used to configure a turbine rotor of a steamturbine, for example, as one method (double melt), the raw material issubjected to vacuum induction melting (VIM) and electro-slag remelting(ESR) and then poured into a prescribed mold. Subsequently, a forgingtreatment and a heat treatment are performed to produce the turbinerotor. As another method (double melt), the raw material is subjected tovacuum induction melting (VIM) and vacuum arc remelting (VAR) and thenpoured into a prescribed mold. Subsequently, a forging treatment and aheat treatment are performed to produce the turbine rotor. As stillanother method (triple melt), the raw material is subjected to vacuuminduction melting (VIM), electro-slag remelting (ESR) and vacuum arcremelting (VAR) and then poured into a prescribed mold. Subsequently, aforging treatment and a heat treatment are performed to produce theturbine rotor. The turbine rotors produced by the above methods areinspected by ultrasonic inspection or the like.

It is described below that the Ni-base alloy according to the presentinvention is excellent in forgeability.

(Evaluation of Forgeability)

It is described below that the Ni-base alloy having the chemicalcomposition ranges of the present invention has excellent forgeability.Table 1 shows chemical compositions of Sample 1 to Sample 5 used forevaluation of the forgeability. And, Sample 1 to Sample 4 are Ni-basealloys with the chemical composition ranges of the present invention,and Sample 5 is an Ni-base alloy with its composition not within thechemical composition ranges of the present invention and used as acomparative example. Sample 5 has a chemical composition correspondingto a conventional steel Inconel 617. The Ni-base alloy with the chemicalcomposition ranges of the present invention contains Fe (iron), Cu(copper) and S (sulfur) as unavoidable impurities in addition to Si andMn.

TABLE 1 (Wt %) Ni C Si Mn Cr Fe Al Mo Co Cu Ti B S Re Example Sample 1Balance 0.099 0.55 0.57 23 1.56 1.21 8.9 19.6 0.24 0.36 0.0039 0.0010.12 Sample 2 Balance 0.099 0.55 0.57 27.4 1.56 1.21 8.9 14.5 0.24 0.360.0039 0.001 2.48 Sample 3 Balance 0.096 0.53 0.57 25.7 1.55 1.2 10.312.2 0.24 0.35 0.0041 0.0009 0.13 Sample 4 Balance 0.096 0.53 0.57 23.21.55 1.2 11.9 12.2 0.24 0.35 0.004 0.0009 2.47 Comparative Sample 5Balance 0.076 0.51 0.55 22.9 1.57 1.21 8.9 12.2 0.25 0.36 0.0038 0.00090 Example

For evaluation of forgeability, Ni-base alloys of Sample 1 to Sample 5having the chemical compositions shown in Table 1 each in 10 kg weremelted in a vacuum induction melting furnace, and test specimens made ofcylindrical ingots having a diameter of 87 mm and a length of 140 mmwere produced. Subsequently, the ingots were undergone a soakingtreatment at 1050° C. for five hours. Forging treatment was conducted bya 500-kgf hammer forging machine at a temperature range of 950 to 1100°C. (reheating at 1100° C.). For the forgeability, the above-describedforging treatment was performed until the test specimens came to have adiameter of 30 mm. Evaluation was performed based on a forging ratio ofthe above treatment and the presence or not of a forging crack at thattime.

The forging ratio is defined by the division of a length of the testspecimen which is a forged object stretched by the forging treatment bya length of the test specimen which is the forged object before theforging treatment. According to the forging treatment, if thetemperature of the test specimen lowers, namely if the test specimenbecomes hardened, the forging treatment is repeated by reheating up to areheating temperature of 1100° C. And, for the presence or not of aforging crack, the test specimens undergone the forging treatment arevisually checked. If there is no crack, it is indicated as “None”, andthe forgeability is evaluated as “0” to indicate that the forgeabilityis excellent. Meanwhile, if there is a crack, it is indicated as “Yes”,and the forgeability is evaluated as “X” to indicate that theforgeability is inferior.

Table 2 shows results obtained by evaluating the forgeability of therespective samples.

TABLE 2 Forging ratio Crack Forgeability Example Sample 1 6.6 NONE ∘Sample 2 6.3 NONE ∘ Sample 3 6.7 NONE ∘ Sample 4 6.4 NONE ∘ ComparativeSample 5 5.4 YES x Example

As shown in Table 2, it was found that Sample 1 to Sample 4 haveexcellent forgeability in comparison with Sample 5.

Although the invention has been described above by reference to theembodiments of the invention, the invention is not limited to theembodiments described above. It is to be understood that modificationsand variations of the embodiments can be made without departing from thespirit and scope of the invention.

1. An Ni-base alloy for a turbine rotor of a steam turbine, the Ni-basealloy contains in percent by weight C: 0.05 to 0.15, Cr: 22 to 28, Co:10 to 22, Mo: 8 to 12, Al: 0.8 to less than 1.5, Ti: 0.1 to 0.6, B:0.001 to 0.006, Re: 0.1 to 2.5, and the balance of Ni and unavoidableimpurities.
 2. The Ni-base alloy for a turbine rotor of a steam turbineaccording to claim 1, wherein the unavoidable impurities are suppressedin percent by weight to Si: 1 or below and Mn: 1 or below.
 3. A turbinerotor configured to dispose through a steam turbine into whichhigh-temperature steam is introduced, wherein at least a predeterminedportion is formed of the Ni-base alloy for a turbine rotor of a steamturbine according to claim
 1. 4. A turbine rotor configured to disposethrough a steam turbine into which high-temperature steam is introduced,wherein at least a predetermined portion is formed of the Ni-base alloyfor a turbine rotor of a steam turbine according to claim 2.